Process for synthesizing luminescent material



g- 6, 1940. H. w. LEVEREQNZ 2,210,087

PROCESS FOR SYNTHESIZING LUMINESCENT MATERIAL Original Filed Jan. 23, 1934 'l/l/l/ \.1\\ F 1. 2 81-02 77 8] mTL mnL'0 INVENTOR HumboldEWLeverenz BY n'r'ron EY Patented Aug. 6, 1940 UNITED STATES PROCESS FOR- SYNTHESIZING LUMINESCENT MATERIAL I Humboldt W. Leverenz, South Orange, N. J., as-

signor to Radio Corporation of America, a corporation of Delaware Application January 23, 1934, Serial No. 707,866

' Renewed July 12, 1939 6 Claims. (01. 250-81) 7 My invention relates to an improved process for synthesizing luminescent willemite which is particularly adapted for making the fluorescent screens in cathode ray tubes for television reception.

6 For obtaining the best results, it has been de termined that material of the character referred to should havethe'following characteristics. It should emit light of a color pleasing to the eye.

10 It should have an efliciency of 2.5 candle power/watts, or more, at 10,000 volts and 100 microamperes, when viewed from the side oDD site to that scanned by the cathode ray. When viewed directly, that is, from the same side as that scanned by the ray, the efdciency should be 5 candle power/watts, or more. The material should decay in brilliancy at a very rapid rate, and at least at the rate of one-thirtieth of a second from extreme brilliance to darkness or go black. The material should have an invariant spectral distribution. It should resist burning" by high-speed electrons. It should have high secondary-emission efficiency. The material should be capable of application to the wall structure of the tube to obtain a uniform and strong screen for observation on the side opposite that which is scanned by the cathode ray. It should begin to fluoresce at a relatively low voltage. It

should withstand use in high vacuum at tempera- 3o tures at least up to 550 degrees centigrade.

I use the term luminescent to describe my material since it is generic to the process of converting radiant energy of a given wave length to radiant energy whose wave length is within the a visible spectrum. It consequently covers both the phenomena of fluorescence and phosphorescence; Fluorescencevis obtained when the conversion time is on the order of 10- seconds or less and represents the time it takes for an ejected electron to change from one orbit to an-' other in accordance with modern atomic theory. Phosphorescence is the emission of light which persists after a time duration of 10- seconds following the excitation of the luminescent materials and in this connection, the theory has postulated that ejected electrons from the material wander in the interstices among the atoms before returning or being recaptured by the atom. Upon its recapture the change in energy between 50 the two levels or electron orbits is indicated by the presence of radiation.

While a synthetic luminescent willemite to meet the above requirements has been obtained heretofore, the methods proposed have necessitated u the me of dangerous hydrofluoric acid and excharacteristics mentioned above, which does not 10 require the use ofv hydrofluoric acid or expensive platinum ware in its production, and which affords as homogeneous a mixture of all the 0on stituents as the division of the silica (S102) or other corresponding material will ail'ord. It will 15 be understood that in the above formula, the

semi-colon indicates that the phosphor ZnaSi04 is activated by the metal Mn. That is to say that the semi-colon merely indicates that a small amount of the elements following the semig0 colon acts as anactivator and is held in physical bolnd with the material'which precedes thesemico on.

' Other objects and advantag s will hereinafter appear. :5

In accordance with my invention, a solution of a metallic salt, such as zinc nitrate, is formed. To this solution, minute particles of a. dioxide, such as silicon dioxide are added, sometimes in colloidal solution. The metal is then precipitated in the form of an insoluble metal compound to cause some of the metal compound to adhere to the individual particles of the dioxide in the form of a layer or coating about the particles as nuclei or cores to form a homogeneous mixture of the dioxide and the compound. The resulting mixture is then washed and dried, and is finally heated to decomposethe insoluble metal compound so that there only remains the homogeneous mixture, each particle of which is substano tially the same and-comprises the metal and the dioxide. It is, understood that the word homogeneous" should be qualified as being macroscopically homogenous. That is in any single particle of the mixture the respective constitucuts thereof bear the same relation to each other as the quantity and relative positions in any other particles. Strictly speaking the material is neither heterogeneous nor homogeneous.

My invention resides in the improved process of the character hereinafter described andclaimed.

In the drawing, Figure 1 is a '-pictorialillus.- tration of the action whichit is understood takes place in-tlie course of carrying out my improved Fig. 2 is an elevational, sectional view, illustrative of one step in a modification of my improved of zinc. Specifically, in one particular operation, a satisfactory batch of the luminescent material was obtained by using 45.6 grams of zinc and 0.23 gram of manganese. In this case, the manganese was used to the extent of being 0.006 gram-mole with respect to one gram-mole zinc. In the particular operation referred to, the solution was obtained by using 1.5 moles of pure, concentrated nitric acid and 200 cubic centimeters of distilled water. This formed 220 to 230 grams of nitrates of zinc and manganese, in solution.

As alternatives in this step, it is proposed to use, instead of the zinc, any other suitable metals such as magnesium, calcium, beryllium, or any other suitable metal in the first three groups of the periodic system and to use, especially, the alkaline-earth metals in group 2 of this system.

Second step.Finely divided, purified silicon dioxide (silica) is added. The particles of silicon dioxide are very small, preferably less than 100 microns in diameter. These particles may be obtained, for example, by grinding in a ball mill, or the like, and then passing the material through a 400 mesh screen. It is proposed to add the silicon dioxide in about 0.45 to 0.6 gram-molecular ratio to the zinc, and touse about a 5% to excess of the silicon dioxide. In the particular operation referred to, the silicon dioxide was added in about a 0.5 gram-molecular ratio to the zinc. The combination is mixed well.

As an alternative in this step, it is proposed to add the silicon dioxide in the form of colloidal silicon dioxide.

As a further alternative, it is proposed to use germanium dioxide instead of the silicon dioxide.

Thirdstep-The zinc and manganese are precipitated as carbonates, oxalates, sulphides, hydroxides or phosphates. When the metals are precipitated as carbonates, this is done by adding sufllcient ammonium carbonate for this purpose, or by making the solution slightly alkaline with ammonium hydroxide and then saturating the same with carbon dioxide. In the particular operationreferred to, ammonium carbonate was used in this step, 93 grams of this compound being dissolved in 500 cubic centimeters of distilled water. It is proposed to heat the solution to hasten the action.

This causes the zinc and manganese to precipitate out of solution as the insolublecarbonates, the remaining liquid containing only ammonium nitrate.

In the precipitating action, it is understood that the minute silicon dioxide particles serve as nuclei about which the zinc and manganese carbonates agglomerate. That is, it is understood that each silicon dioxide particle serves as a core to which there adheres at least one layer or coating of precipitated carbonates of zinc and manganese. This is represented at the left, in Fig. 1

last washing liquor is decanted, and the remaining mass is dried.

Fifth step.The dried material is then heated in a refractory crucible at a temperature from 900 degrees to 1400 degrees centigrade and for a period from 5 to 150 minutes, depending upon the amount of the material and the characteristics of the heating furnace, as will be well understood. When the zinc and manganese are precipitated as sulphides, it is proposed to carry out this heating in an atmosphere containing oxygen. This decomposes the sulphides and removes the sulphur as sulphur dioxide. In view of the fact that CO2 is driven 011 during the final heating step, it might very naturally be inferred that the resulting compound would be a mixture of zinc and manganese silicates. Such is apparently not the case, since the final material seems to be a zinc orthosilicate wherein manganese is entrained as an activator. The formula, ZnzSiOgMn, therefore,

.has been assigned to my improved synthetic willemite but it is to be definitely understood that I am not to be bound by any particular theory of formation thereof.

Where germanium dioxide is used instead of silicon dioxide, the final result is to obtain a luminescent material known as zinc orthogermanate (ZmGeOuMn). Alternatively any one of the other elements in group IV of the periodic table may be used in place of silicon or germanium.

As an alternative, it is proposed to replace the zinc by cadmium up to the extent of about Also, in the first step of dissolving the zinc and manganese or any other metal into the. aqueous solution, theefliciency of secondary emission may be increased by adding a small amount of a suitable substance, such as barium, strontium, caesium, cerium, thorium, rubidium, etc. Such a substance can also be added after the carbonates have been formed, such as after the decantation of the last washing water in the fourth step.

Instead of adding the dioxide as minute particles thereof, as in the second step above, the dioxide may be placed into the nitrate solution of the first step in the form of a piece [0 of the dioxide, as represented in Fig. 2. This piece may be in the form of a relatively thin fiat sheet or an irregular piece. In such case, the precipitating action, as in the third step above, causes the zinc and manganese carbonates, for example, to be precipitated over the submerged surface of the dioxide material as a layer or coating i 2. A cross section of the material would then appear as shown at the left in Fig. 3, with the silicon or germanium dioxide in the center and the outer coating of zinc carbonate and manganese carbonate. The sheet or piece of the dioxide material, with the coating of zinc and manganese carbonates is then removed from the nitrate solution and washed and dried, corresponding to the fourth step above. Heat is then applied, corresponding to the fifth step above, which causes decomposition of the zinc and manganese carbonates to the oxides of these metals, and penetration in some fashion into the dioxide material to bring about the transition as pictorially illustrated in Fig. 3. The result is that there will be a substantial layer of the willemite (ZnzSiOgMn) over a core of the-uncombined dioxide material. The willemite is then scraped off for use. In this connection, however, it is proposed to start with a flat sheet or a polished surface of the dioxide material, and to submerge and coat only the polished surface or one'side ofv the sheet with the carbonates. The decomposition (fifth step) is then carried out to produce a uniform layer of the willemite material, which can be used directly as the fluorescent screen for a cathode ray tube. For example, the flat screen may be made to form the large end wall of a cathode ray tube, with the screen surface on the inside, facing the electron gun. I

By my improved process I obtain a luminescent willemite which satisfactorily meets all the requirements referred to for a cathode ray tube in television reception, and which can be made without the use of dangerous hydrofluoric acid and expensive platinum ware. Most important, also, is the fact that by my improved process I obtain good luminescent willemite in which there is as homogeneous a mix of all the constituents as the division of the silicon dioxide or the germanium dioxide will afford. As explained, this division can be made so small that the dioxide particles can be subdivided to practically molecular dimension. By my improved process, I eliminate the requirements for use of any halide.

I claim as my invention: a

1. The steps in the process of making manganese activated luminescent material which comprise forming salts of zinc and a small amount of manganese in solution, adding to the solution minute particles of silica, precipitating the zinc and manganese out of the solution as insoluble salts to cause crystallization of the insoluble salts about the individual particles of silica, washing and then drying the resulting mass, and heating the dried mass to decompose the insoluble metallic salts to oxides of the metals and to chemically combine the zinc oxide with the silicon with man-' ganese entrained as an activator.

2. The process of making manganese activated zinc orthosilicate (ZnzSiOgMn) which ,c'oinprises precipitation of insoluble salts of zinc and a small amount of manganese onto the surface of silicon dioxide, and then decomposition by heat of the said zinc and manganese salts into oxides of these metals and further heating said oxides to form with the silicon dioxide a macroscopically homogeneous manganese activated zinc orthogermanate (ZmGeOgMn) which comprises precipitation of insoluble salts of zinc and a small amount of manganese onto the surface of germanium dioxide, and then decomposition by heat of the said zinc and manganese salts into oxides of these metals and further heating the resultant mass to form with the germanium dioxide a macroscopically homogeneous manganese activated zinc orthogermanate.

4. The process of making luminescent material which comprises obtaining a solution of salts of zinc and manganese, the manganese being used to the extent of from 0.0001 gram-mole to 0.01 gram-mole with respect to one gram-mole of zinc, addition to the solution of minute particles of a dioxide of an element in the fourth group of the periodic. system, precipitation of insoluble salts of the zinc and manganese into adherence with the individual particles of the dioxide, decomposition by heat of the said insoluble salts of zinc and manganese into oxides of these metals and further heating the resultant mass to form with the dioxide macroscopically homogeneous luminescent material.

5. The process of making manganese activated luminescent material which comprises formin a solution of a salt of a metal chosen from the first three groups of the periodic system anda small amount of a salt of manganese, adding to the solution minute particles of a dioxide of an element in the fourth group of the periodic system, precipitating the manganese and the metal inthe form of insoluble compounds to cause the same to adhere to the individual particles of the dioxide in the form of a coating about said particles as cores, said insoluble compounds being capable of decomposition by heat into an oxide of manganese and an oxide of the metal, washing and then drying such mixture, heating said mixture to decompose the insoluble compounds into an oxide of manganese and an oxide of the metal, and further heating the resultant mass to chemically combine theoxide of the metal with the dioxide and to entrain the manganese as an activator. v v

6.The process of making a manganese activated luminescent material which comprises the steps of precipitating from a solution a salt of a metal from the first three groups of the periodic system onto the surface of a dioxide of an element in the fourth group of the periodic system and a small amount of a salt of manganese, and then decomposing by heat the precipitated salts into an oxide 'of manganese and an oxide of metal, and heating said oxides to form with the dioxide a macroscopically homogeneous luminescent material activated by manganese.

HUMBOLDT W. LEVERENZ. 

